Origin of the n-type and p-type conductivity of MoS2 monolayers on a SiO2 substrate

0.72 MB·English

by  Kapildeb Dolui

Origin of the n-type and p-type conductivity of MoS monolayers on a SiO substrate
2 2
Kapildeb Dolui, Ivan Rungger and Stefano Sanvito
School of Physics and CRANN, Trinity College, Dublin 2, Ireland
(Dated: January 14, 2013)
Ab-initio density functional theory calculations are performed to study the electronic properties
of a MoS monolayer deposited over a SiO substrate in the presence of interface impurities and
2 2
defects. When MoS is placed on a defect-free substrate the oxide plays an insignificant role, since
2
theconductionbandtopandthevalencebandminimumofMoS arelocatedapproximatelyinthe
2
middleoftheSiO band-gap. However,ifNaimpuritiesandOdanglingbondsareintroducedatthe
2
SiO surface,theseleadtolocalizedstates,whichmodulatetheconductivityoftheMoS monolayer
2 2
from n- to p-type. Our results show that the conductive properties of MoS deposited on SiO are
2 2
3
mainly determined by the detailed structure of the MoS /SiO interface, and suggest that doping
1 2 2
the substrate can represent a viable strategy for engineering MoS -based devices.
0 2
2
PACSnumbers:

n
a
J I. INTRODUCTION for GaAs nanowires it has been demonstrated that upon
1 decreasingthenanowirediametertheinterface-mediated
1 conductivity gradually becomes dominant over the bulk
Recently MoS -based layered transition metal chalco-
2 one18. Such surface sensitivity can also be used to one’s
genides (LTMDs) have attracted considerable attention
] advantage. Forinstanceanambipolartransistorhasbeen
i due to their potential for constructing low dimensional
c realizedinMoS thinflakesbycontactwithaionicliquid
nano-structuresforavarietyofapplications1–3. Theelec- 2
s environment19, which as well affects the interface prop-
- tronic properties of MoS show a strong dependence on
l 2 erties. Since MoS monolayers are placed on insulators
r thickness4, i.e. on the number of atomic layers forming 2
t in practically any device architecture, it is important to
m a given sample. In particular, MoS2 monolayers, which
identifythepossibleeffectsthatthesubstratehasonthe
display a substantial direct band-gap, represent a semi-
. conductivity.
t conducting alternative to graphene, which is a metal in
a
m itspristineform. Althoughtheband-gapofgraphenecan The defects responsible for the conductive properties
be opened by fabricating nanoribbons5 or by depositing of low dimensional devices are expected to be extrin-
-
d it on a suitable substrate6, this comes to the prize of de- sic in nature, such as charged impurities at the inter-
n teriorating, in a somehow uncontrollable way, the carrier face between the conductive channel and the substrate.
o mobility due to edges and impurity scattering7. In con- These lead to an inhomogeneous Coulomb potential for
c trast, the low dimensionality, the small amount of dan- bothconductionandvalencebandelectrons. Suchcharge
[
glingbonds,andtheirtypicalhighcrystallineform,make trapshavebeenidentifiedtobeintheformofadsorbates

1 theperformancesofLTMD-basedtransistorscomparable or defects at the surface of the underlying substrate in
v tothoseofexistingSi-basedones8–10. Inparticulartran- the case of graphene20,21. Likewise, temperature depen-
1
sistors made from MoS monolayers have been recently dent transportmeasurementson thinMoS layers, down
9 2 2
fabricated,showingamobilityofatleast200cm2/V·sat to the monolayer limit, suggest that trapped charges at
4
roomtemperature,anon/offcurrentratioof108 andlow the SiO surface could be responsible for the observed
2 2
. standby power dissipation11. n-type behavior, when MoS2 is deposited on SiO222.
1
0 Interestingly,bothn-type11–14 andp-type15,16 conduc- In general when charged traps are located at an inter-
3 tivities have been reported in ultra-thin MoS2 layers de- face, they influence the depletion/accumulation of elec-
1 positedonSiO2. TheconductingbehaviorofMoS2there- tronsintheconductingchanneluptoacertainthickness,
: fore seems to depend on the experimental details and which is proportional to the channel screening length.
v
i an explanation for the specific current polarity (n- or Thisdistancedependsondifferentphysicalfeatures,such
X p-type) remains far from being clear. Note that no in- as the nature and the density of the traps, and the
r tentionaldopingwasintroducedintheexperimentsmen- electronic properties of the channel. For conventional
a
tioned above, so that the source of the different carrier semiconductors it typically reaches up to a few nanome-
types should be intrinsic to the MoS2 layer, to the sub- ters. Forinstanceithasbeenrecentlydemonstratedthat
strate and to the interaction between the two. chargedtrapstatesatthesubstrate/channelinterfacesig-
The possible creation of Mo and/or S vacancies dur- nificantly affect the conductivity of GaAs nanowires up
ing the growth cannot be the cause of the various con- todiametersofabout40-70nm18. Moredramaticeffects
ductive properties, since vacancies create deep levels at are expected for layered compounds down to the single
mid-gapinthebandstructureofMoS monolayers17. No- layer limit, in which essentially all the atoms are at the
2
tably, disorder at the semiconductor/substrate interface interface with the substrate and the channel vertical di-
in general plays a crucial role in determining the con- mension is certainly shorter than the screening length.
ductive properties of ultra-thin devices. For example, Recently a reduction in conductivity with increasing the
2
MoS film thickness has been observed in MoS -based lations such mechanisms depends crucially on the ability
2 2
transistors, where SiO was used as back gate23. This of computing accurately the energy levels of the system.
2
suggests that for the MoS /SiO system the transport is The use of the GGA (or of the local density approxima-
2 2
interface-mediated, as intrinsic defects, homogeneously tion-LDA)forelectronicstructurecalculationsofdefect
distributed in MoS , would not lead to any dependence levels is, in general, problematic. One reason is the typ-
2
of the conductivity on thickness. ical underestimation of the energy gap and the related
In order to shed some light on the effects that trap incorrect alignment of the energy levels of hybrid sys-
states at the SiO surface have on the conductive prop- tems. For instance an artificially reduced band-gap may
2
erties of MoS /SiO hybrid systems, we have performed erroneouslybringdeeptrapsinresonancewitheitherthe
2 2
state of the art first principle electronic structure calcu- conduction or the valence band30,31. A second source of
lations. In particular we have considered the case when erroristheincorrectdescriptionofthechargelocalization
the traps are due to impurities such as immobile Na and at the defect site, a feature that usually leads to predict
H atoms, and O-dangling bonds. The paper is organized defects to be too shallow32. Atomic self-interaction cor-
as follows. In the next section we briefly describe our rection (ASIC)33,34 has been proved to overcome these
computational techniques and we provide details of the deficiencies35,36. Therefore we also perform additional
simulationsperformed. Thenweproceedwithpresenting LDA+ASIC calculations to verify the robustness of the
the results of this work in the context of recent experi- LDA/GGAresults. InparticularwesettheASICscaling
ments, and finally we conclude. parameter to α = 0.5, a value which is generally appro-
priate to mid-gap insulators33.
II. METHODOLOGY
III. RESULTS AND DISCUSSION
InordertoinvestigatetheinfluenceofaSiO substrate
2
on the electronic properties of a MoS2 monolayer, ab- A. Defect-free SiO2 interface
initio calculations are performed by using density func-
tional theory (DFT)24,25 within the generalized gradient Substantial experimental efforts have been devoted to
approximation (GGA) of the exchange and correlation deposit ultra-thin MoS layers onto SiO in order to
2 2
(XC) functional as introduced by Perdew, Burke and demonstrate transistor operation, down to the single
Ernzerhof (PBE) 26 and numerically implemented in the layer limit11,12,16. Usually amorphous oxides are used as
SIESTA code27. In our calculations, a double-ζ polar- substrates. However,inordertoavoidthecomputational
ized28 numerical atomic orbital basis set is used for all complexity of a highly disordered structure, a crystalline
the atoms and the Troullier-Martins scheme is employed SiO substrate is simulated here instead. This also al-
2
forconstructingnorm-conservingpseudopotentials29. An lows us to systematically determine the effects of indi-
equivalentplanewavecutoffof350Ryischoseninallthe vidual defects and impurities on the electronic structure
simulations and the Brillouin zone is sampled by using of a MoS layer. Our unit cell is constructed as a slab
2
an equivalent k-grid cutoff of 17 ˚A. Relaxed geometries containing at least 6 Si atomic layers of α-quartz and an
are obtained with the conjugate gradient method, where adsorbed MoS monolayer. At least 15 ˚A of vacuum are
2
all the atoms in the supercell are allowed to relax until included at the slab boundaries to avoid the spurious in-
the force on each atom is less than 0.02 eV/˚A. teraction between the slab periodic images. We consider
Atrapstateisusuallyformedwhenanenergylevelas- the modified oxygen-terminated (0001) SiO surface in
2
sociated either toa defect or an impurity appears within order to simulate the most experimentally relevant con-
the energy gap of the host material. Such trap states ditions.
influence the charge transport properties mainly in two Two primary structures for the oxygen-terminated
ways. Firstly, if the traps are charged, they will capture SiO (0001) surface are possible, depending on whether
2
a hole or an electron from the environment. This pro- the termination is with the siloxane group (Si-O-Si) or
ducesamodificationoftheelectrostaticpotential, which with the silanol one (Si-OH). Both surfaces can form de-
in turns shifts the level alignments in the system, and pending on the surface treatment37. The siloxane recon-
thus affects the conductivity. Secondly, they can also in- struction at room temperature forms an O-terminated
creasethecarrierconcentrationandprovidepathwaysfor surface with an outermost six-membered ring structure,
electrons or holes to hop. The efficiency of this process asshowninFig.1(a,b). Underannealinginambientcon-
dependsontheamountoflocalizationofthestates asso- ditions it becomes hydroxylated (Si-OH) and the recon-
ciatedtothedefectsite. Iftheenergyofthelocalizedgap struction transforms into the silanol one, which presents
stateisclosetoeitherthevalencebandmaximum(VBM) on the surface a zigzag H-bonded network [see Fig. 1(c,
ortheconductionbandminimum(CBM),thenatagiven d)]. In both cases in our simulations the dangling bonds
temperature some of these charges will be transferred ei- ontheSi-terminatedbottomsurfacearesaturatedbyhy-
thertotheconductionortothevalenceband,wherethey drogen.
may contribute to increase the system conductivity. The optimized lattice constants of the pristine SiO
2
Whether or not one can describe with ab initio calcu- andMoS are4.91˚Aand3.19˚A,respectively. Wethere-
2
3
(a) (c) ilar to the distance between two MoS monolayers that
2
we calculate to be 3.17 ˚A. The binding energy of the
MoS /SiO system is given by E = E + E -
2 2 b MoS2 SiO2
E , where E , E , and E are to-
MoS2/SiO2 MoS2 SiO2 MoS2/SiO2
tal energies of the isolated MoS , the isolated SiO slab,
2 2
and of the MoS /SiO hybrid system, respectively. We
2 2
findE forsiloxaneandsilanoltoberespectively0.14eV
b
and 0.16 eV per primitive MoS unit cell. These binding
2
energies are close to that between two MoS layers (0.20
2
eV/unit cell), which are bound together by the rather
(b) (d) weak van der Waals forces. As such, our results show
that MoS is weakly bound also to the SiO surface,
2 2
in agreement with recent experimental results that have
measured the interaction between MoS and an underly-
2
ing SiO substrate to be negligible39.
2
Note that in general GGA-type XC-functionals do not
describe accurately van der Waals forces. However, it
has been shown that the LDA/GGA is able to repro-
FIG. 1: (Color online) Side and top views of reconstructed
structures for the O-terminated SiO (0001) surface (a and duce the interlayer spacing and the binding energy of
2
b), and the fully hydroxylated SiO (0001) one (c and d) layered chalcogenides40. We have then verified that our
2
(color code: cyan → Si, red → O, violet → H). calculated d for bulk MoS , d = 3.08 ˚A, is in good
0 2 0
agreement with the experimental value of 2.96 ˚A41 and
also with the previously calculated theoretical estimate
of3.05˚A42. Moreover, inordertotakeintoaccountpos-
sible small deviations of the relaxed distance from the
experimental value due to the XC functional used, we
have evaluated the electronic structure for d within the
0
range d ±0.5 ˚A, and we have found that the results
0
(a) (b) change little with varying the distance.
FIG. 2: (Color online) Top view of the optimized structure
of MoS2 placed on a defect-free (a) siloxane and (b) silanol B. SiO2/MoS2 composite with siloxane
surface(colorcode: lightgrey→Mo,yellow→S,cyan→Si, reconstruction
red → O, violet → H).
We now move to study the electronic structure of a
MoS monolayer deposited on SiO by starting with the
2 2
fore construct a hexagonal supercell in the plane, with a siloxanesurface. Inparticularweconsiderfirstthesitua-
9.69 ˚A-long side, so that the lattice mismatch between tionwhereSiO isdefect-free. Fig.3(c)showsthedensity
2
SiO2 and MoS2 is minimized to ∼1.2 %. The GGA cal- of states (DOS) of the hybrid SiO2/MoS2 system, which
culated band-gap of SiO2 and of a MoS2 monolayer are remains semiconducting with a band-gap of 1.48 eV, i.e.
6.20 eV and 1.49 eV, respectively. The small strain ap- with the same band-gap of a free-standing MoS mono-
2
pliedtotheMoS2 monolayerchangesonlylittletheelec- layer having the same lattice parameters. Both the va-
tronic structure. The band-gap remains direct at the K lence and the conduction bands of the hybrid compound
pointand itisonly reducedby 0.22eVfrom thevalueof areassociatedtoMoS . WenotethattheprojectedDOS
2
1.71eVobtainedfortheunstrainedcase. Similarlytothe (PDOS)overMoS extendsinto theSiO band-gap, and
2 2
case of graphene38, we expect that the electronic struc- the total DOS of the combined material is essentially
ture of a MoS2 monolayer is only marginally affected by given by the superposition of the DOSs of the pristine
its local arrangement on the SiO2 substrate. Therefore, slab of SiO2 [Fig. 3(a)] and of the MoS2 monolayer
asarepresentativeconfigurationweusethearrangement [Fig. 3(b)]. Both the conduction and the valence bands
ofFig.2,whereanoxygenatomissituatedatthehollow of SiO are located at least 1.5 eV away from those of
2
site of the Mo surface triangles. MoS . Asaconsequence, nochargetransferbetweenthe
2
We start our discussion by presenting the properties substrate and MoS occurs. Importantly, one of the ba-
2
of the defect-free hybrid MoS /SiO system. The equi- sic criteria for the selection of the gate oxide is fulfilled
2 2
librium distances, d , between the SiO and the MoS here, namely that the oxide should have a bands offset
0 2 2
surfacesare3.01˚Aand2.98˚Aforsiloxaneandsilanol,re- of over 1 eV for both the conduction and valence band
spectively. Here d is defined as the vertical separa- in order to create a large barrier for both electrons and
0
tion between the top-most O layer in the SiO surface holes43. Our results show that the conductivity of MoS
2 2
and its nearest S layer in MoS . These values are sim- is not influenced by the underlying defect-free SiO sub-
2 2
4
Defective Defective
30 SNiaO-S2iO2 (a) SiO2 MoS 2 (b) SiO2 MoS2 (c) SiO2 MoS2
(a) H-SiO2 Evac Evac Evac
0
EC1 EC1 EC1
-30 EF
30 (b) MoS2 EF EC2 EC2 EC2
0 EV2 EV2 EF EV2
-30
es) EV1 EV1 EV1
V/stat 300 (c) TMootaSl2
OS (e-30 pFlIaGce.d4:on(CthoelodreofencltinSei)OS2chsuebmsatrtaicteb(aan)d, adniadgoranmafsourbsMtroaSt2e
D 30 (d) Total including a defect-induced donor (b) or acceptor (c) level.
MoS2 This demonstrates the modulation of the conductivity from
0
n-typetop-typeastheimpuritystateredefinestheFermien-
-30 ergy in the oxide. The energy levels E , E , E and E
V C F vac
30 (e) Total define the valence band maximum (VBM), the conduction
MoS2 band minimum (CBM), the Fermi energy, and the vacuum
0
level, respectively. The subscripts 1 and 2 refer to SiO and
2
-30 MoS ,respectively. Thebluedashed-linesindicatetheFermi
2
-4 -2 0 2 4
energy of the hybrid system (common to SiO and MoS ).
E - E (eV) 2 2
F The thick green line in (b) indicates the donor level and the
thickredlinein(c)representstheacceptorstateintheoxide.
FIG.3: (Coloronline)ElectronicstructureoftheSiO /MoS
2 2 NotethatingeneralduetochargetransferfromMoS tothe
2
hybrid system when various defects are present at the SiO
2 gap states, and the related dipole formation, the level align-
siloxane surface. (a) The total DOS for the defect-free sur-
ment between E and E will also change in the defective
V1 V2
face(black,dashedcurves),andwheneitherNa(green,solid
systems.
curves), orH adsorbed(magenta, solidcurves)are adsorbed.
(b)TheDOSofapristinefree-standingMoS monolayer. The
2
total DOS and the PDOS for MoS , when the MoS mono-
2 2
layer is placed on the defect-free siloxane surface (c), on a
siloxane surface with one adsorbed Na, or (d) with one ad-
sorbed H. The blue dashed vertical line indicates the Fermi
level, which has been set to zero in all the panels. The red
shadedareasindicatetheMoS PDOS.Positiveandnegative
2
DOS are respectively for spin up (majority spins) and spin
(a) (b)
down (minority spins) electrons.
FIG. 5: (Color online) The optimized geometry for MoS
2
placed on (a) the siloxane surface incorporating a Na impu-
strate. Thereforethemeasuredn-typeorp-typeconduct- rityand(b)thedanglingoxygenbondonthesilanolsurface,
ing properties of MoS2 on SiO2 must be due to defects obtainedbyremovingaHatom. Thearrowsindicatethepo-
and impurities. sitionsofthedefectsonthesurface(Colorcode: green→Na,
Localized states, arising from impurities or defects while the other colors are the same atoms as in Fig. 2). The
within the oxide substrate or at the interface with the arrows indicate the location of the impurities/defects.
conductingchannel,canredefinetheeffectiveFermilevel
of the hybrid system, as illustrated schematically in
Fig.4. Dependingonthealignmentofthegapstateswith possible candidates: Na atoms and SiO2 surface oxygen
respect to the MoS valence and conduction bands, the dangling bonds.
2
system can switch from n-type [see Fig. 4(b)] to p-type Duringthesynthesisandthesamplepreparation,SiO
2
[see Fig. 4(c)]. Therefore, such trap states are expected canadsorbrelativelylightimpuritiessuchasNaandK46
to give significant contributions to the conductivity of atitssurface. Inordertosimulatetheeffectsofsuchim-
these low dimensional systems. In the layered structure purities on the electronic structure of the MoS channel
2
considered in this work, the trap states are expected to a single Na atom is placed on top of the siloxane SiO
2
be located at the interface between the LTMDs and the surface. Given the lateral dimension of our supercell,
substrate, not in the LTMDs themselves, which usually this corresponds to an impurity density of ∼1014 cm−2,
are highly defect-free. Trap states at the SiO surface which is close enough to the recently reported values of
2
can have a wide range of origins, such as immobile ionic trapstatesdensities,reachingupto∼1013 cm−2 forthin
charges, SiO surface dangling bonds, and foreign impu- MoS layers deposited on SiO 8,23. The most energeti-
2 2 2
ritiesadsorbedonthesurface44. Inliteraturedensitiesof cally favorable binding position for Na is found to be at
trapstatesonSiO arereportedintherange45 1010-1014 thecenterofthesurfaceoxygentriangle[seepanel(a)of
2
cm−2. As representative dopants, here we consider two Fig. 5]. A Na adatom adsorbed on a pristine SiO sur-
2
5
facecreatesadeepdonorstateintheDOS,withasingle 30 SiO2
(a)
SiO*
particle level at about 2 eV below the SiO CBM [see
2 0
Fig. 3(a)]. Note that such state is singly occupied and
-30
therefore spin-splits in our spin-polarized calculations,
-4 -2 0 2 4 6 8
with the empty minority spin state (spin down) laying 30 Total
(b)
approximately 1 eV below the CBM and 1 eV below the s) MoS2
e 0
Fermi level. at
st
When a MoS monolayer is deposited over the the V/-30
Na-doped SiO2 2surface, d0 increases to 3.24 ˚A at the OS (e 30-6 (c) -4 -2 0 2 4 Total 6
edges of our unit cell, whereas at the Na site the O-S D MoS2
idstinthyiegssettiadeisnnmistcsteetairrsnfobaacnecegcdeolc.iyomremTaecstffphaece3cro.et4ened5lsdeetc˚Aqotbur.ytoehTnnathichcteeeosoeftpfnrtrtulheahcesretegpuneNrrcmieaesetoinionnftfteetothrShfceieaOtlhca2Noet/maiMbopniionooSdans2t--, -330000-6 (d) -4 -2 0 2 4 TMootaSl26
as shown in Fig. 3(c). Also in this case the total DOS -30
appears as a direct superposition of those of SiO and -6 -4 -2 0 2 4 6
2 E - E (eV)
MoS . However the presence of the Na filled state shifts F
2
the Fermi level, which now gets pinned just below the
FIG.6: (Coloronline)ElectronicstructureoftheSiO /MoS
MoS CBM. The resulting DOS around E is thus that 2 2
2 F hybrid system when various defects are present at the SiO
2
of the defect-free MoS conduction band with the addi-
2 silanol-reconstructed surface. (a) The DOS for the defect-
tion of a Na-derived impurity level positioned below it. free surface (black, dashed curve) and for the one where one
Hence, the gap state is moved below the Fermi energy, OdanglingbondisinducedbyasingleHremoval(green,solid
resultinginaverysmallactivationenergyforthetransfer curvelabelledasSiO∗). ThetotalDOSandtheMoS PDOS
2
ofelectronsfromNatotheMoS2 conductionband. This for the SiO2/MoS2 composite when the MoS2 monolayer is
is the situation schematically presented in figure 4(b), placed on (b) the defect-free surface, (c) on the surface with
which leads to n-doping. a single adsorbed Na atom, and (d) on the surface with a
O dangling bond created by removing a single H atom. The
If we now replace Na with H on the SiO siloxane sur-
2 blue dashed line indicates the Fermi energy, which is set to
face, the associated filled gap state lies deep in the SiO
2 zero in all panels. The red shaded areas indicate the MoS
band-gap [see Fig. 3(a)], despite the fact that H and Na 2
PDOS. Positive and negative DOS are respectively for spin
sharethesames-likevalence. Thesamesituationpersists
up(majorityspins)andspindown(minorityspins)electrons.
inthecomposite[seeFig.3(e)],wheretheH-derivedfilled
spin-up level remains at mid-gap, approximately 0.5 eV
abovetheVBM,whiletheemptyspin-downoneisnearly
corresponds to a superposition of the DOSs of the iso-
resonant within the conduction band. This situation
lated MoS [Fig. 3(b)] and SiO [Fig. 6(a)] components,
2 2
howeverdoesnowleadtodopingsothatHcannotinflu-
indicating weak interaction between the two materials.
ence the conductivity of the MoS /SiO structure. The
2 2 When a Na atom is intercalated between the silanol sur-
quantitative difference found between the results for the
faceandtheMoS layer,wefindthatthesystembecomes
2
Na and the H case show that, in order to obtain n-type
n-type[Fig.6(c)],inthesamewayasforthesiloxanesur-
character, only impurities with rather small ionization
face. This indicates that Na is an efficient n-dopant for
potentialarerelevant. Thesecantransferoneelectronto
MoS on SiO regardless of the surface reconstruction.
2 2
the MoS conduction band with small activation energy.
2 Ingeneralthermalannealingofthesilanolsurfacecre-
Such activation energy is a key factor in the determina-
atesunder-coordinatedoxygenatoms(Si-O*). Theseap-
tion of the threshold voltage, V , required to operate a
th pear as stable surface defect centers and act as typical
transistor in the on-state. As a consequence, the exper-
charge traps in oxygen rich SiO 47, since they are able
imentally measured values for V , which show a large 2
th to capture an extra electron in their dangling bond. In
variationfordifferentsamples22,arethenattributableto
our calculations, such defects are created on the Si-OH
varying concentrations and properties of the trap states
surface by removing a H atom from the top surface [see
from sample to sample.
Fig. 5(b)]. For such a defect we find that the empty
acceptor state is created ∼0.9 eV above the SiO VBM
2
[see Fig. 6(a)]. Once MoS is layered onto the surface,
2
C. SiO2/MoS2 composite with silanol the value of d0 at the boundary of our H-deficient unit
reconstruction cell is d =2.98 ˚A, which is approximately equal to that
0
for the pristine surface, whereas at the dangling bond
Next we move to examine the case of the SiO sur- site the O-S distance is significantly reduced to 2.68 ˚A.
2
face with silanol reconstruction, whose DOS is presented When placing the MoS monolayer on this defective sur-
2
in Fig. 6(a). Similarly to the siloxane case, the PDOS face, the dangling bond state gets filled by capturing an
for the defect-free MoS /SiO composite [see Fig. 1(b)] electron from the MoS valence band, so that the Fermi
2 2 2
6
30 (a) Total portantly the ASIC result is rather close to that calcu-
MoS2 lated49 with the hybrid Heyd-Scuseria-Ernzerhof (HSE)
0 exchange-correlationfunctional50. Thisishoweverlarger
es) than the optical band-gap of 1.90 eV measured experi-
V/stat-30-6 -4 -2 0 2 4 mentally for MoS2 monolayers12. The apparent contra-
e diction can be solved by noting that the optical exci-
OS ( 30 (b) TMootaSl2 tations involve excitons with a large binding energy of
D the order of 1 eV, as confirmed by many-body calcula-
-300-4 -2 0E - EF (eV)2 4 6 seGtoipVroWen,scesti4lrnsf9uc-.chmgoeoTnmohshdeiaus,stsaee,angitotrhnlebyeeea5rmn1edaex(n-tp2gte.at7chpw6tesiotefihtfVrhsa)att.pthpAoatrrthsodexecsiroutmmcrluhaepet,vueetlqtlyh4eu9eda1s(A.iw92-S+.pi8tIa12Chr=teitd2cVh.el9ee)-
scribes MoS with a band-gap larger than the measured
2
FIG. 7: (Color online) Density of states for the defective
optical one and it provides an improved description over
SiO /MoS composite calculated with the ASIC XC func-
2 2 that of the GGA.
tional. In panel (a) we report the DOS for the siloxane re-
We now go back to the SiO /MoS composite and in
construction with an intercalated Na atom [corresponding to 2 2
Fig 3(d)], while in (b) that for the silanol reconstruction and Fig. 7 we report two representative results for the case
an O dangling bond obtained by removing a surface H atom of Na adsorbed on the siloxane surface and for that of
[corresponding to 6(d)]. the O dangling bond on the silanol one. We find that
for the first case, although the band-gaps of the two
parentalmaterialsarebothincreased,theFermienergyis
energynowliesjustbelowtheMoS VBM[seeFig.6(d)]. still pinned at the bottom of the MoS conduction band
2 2
This is the level alignment presented in Fig. 4(c), which [Fig. 7(a)]. As a consequence Na still leads to a n-type
makes the composite p-type. Note that the rather high semiconducting character with small activation barrier.
density of oxygen dangling bonds in our system causes a Similarly the O dangling bond on the silanol-terminated
largesurfacechargedensitydipole,whichshiftstheMoS surface leads to a p-type semiconducting character [see
2
DOS downwards in energy by more than 1 eV with re- Fig. 7(b)], with the Fermi energy positioned below the
spect to the SiO substrate. By modulating the density MoS valenceband. Thisindicatesthatourtwomainre-
2 2
of such defect types one may be able to change such a sults remain unchanged whether calculated at the GGA
shift. or ASIC level, i.e. they are robust with respect to the
choice of exchange-correlation functional.
D. Robustness of the results against the choice of
XC functional: ASIC IV. CONCLUSION
Finally, in order to verify that the calculated level The effects of the SiO substrate on the conductiv-
2
alignment is robust against the choice of exchange and ityofasemiconductingMoS monolayerareinvestigated
2
correlation functional, we have repeated our calculations with first principles density functional theory calcula-
by using the ASIC scheme. As expected the ASIC func- tions. The defect-free SiO surface does not affect sig-
2
tional increases the band-gap of MoS and SiO respec- nificantly the electronic properties of MoS due to their
2 2 2
tively to 1.73 eV and 8.02 eV (for the same strained hy- weakmutualinteraction. Assuchtheconductiveproper-
brid structure used in the previous sections). In the case ties of MoS do not change and SiO appear as an ideal
2 2
ofSiO thisbringsthecalculatedvaluesensiblycloserto gate material. However, when Na atoms are placed at
2
the experimental one of 8.9 eV48, as expected from the the SiO /MoS interface, a shallow donor trap state is
2 2
ASIC when dealing with an insulator whose valence and created just below the CBM of the hybrid SiO /MoS
2 2
conduction bands have different orbital content33,34. composite. The small activation energy makes the hy-
The situation for MoS is more complicated and de- bridMoS /Na-SiO systeman-typesemiconductoreven
2 2 2
serves a detailed discussion. In this case the band-gap is for rather low temperatures. Interestingly, the behavior
definedbybandsdominatedmainlybyMo-dorbitalsand is different for H adsorption, where the impurity level is
the ASIC opens it only marginally. For a free-standing created ∼0.9 eV below the CBM, resulting in a stable
MoS monolayer the ASIC (α = 0.5) returns a direct localized charge that cannot be easily promoted to the
2
band-gapof2.03eV(comparedtoaGGAgapof1.71eV). CBM and does therefore not affect the conductivity.
Note that the LDA value is 1.87 eV so that the LDA al- Incontrast,inthecaseofoxygendanglingbondsonthe
ready partially opens the gap with respect to the GGA. silanol-terminated SiO surface, the Fermi energy of the
2
It is also notable that an enhancement of the screening MoS /SiO system is located just below the VBM, mak-
2 2
parameter α to α = 1 (full atomic correction) produces ing the system a p-type semiconductor. These results
a marginal further increase of the gap to 2.10 eV. Im- shows that the conductivity of ultra-thin semiconduct-
7
ing LTMDs changes from n-type to p-type depending on Acknowledgments
the charge-polarity of the traps, as well the energy level
alignmentofthetrapstateswithintheLTMDsbandgap.
ThesekindoftrapstatesattheSiO surfacearelikelyto
2
beattheoriginoftheobservedchangeinconductancein
differentexperimentallyrealizedMoS -basedtransistors.
2
Intriguingly, our results suggest the possibility of inten- This work is supported by Science Foundation of Ire-
tionally doping MoS by depositing different adsorbates land (Grant No. 07/IN.1/I945) and by CRANN. IR ac-
2
over the substrate SiO surface. This can pave the way knowledges financial support from the King Abdullah
2
for a new strategy in the design of two-dimensional de- University of Science and Technology (acrab project).
vices, where the electronic properties of the channel are WethankTrinityCentreforHighPerformanceComput-
engineered by manipulating those of the substrate. ing (TCHPC) for the computational resources provided.
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